The present disclosure is directed to an accessory for use with a standard engine lathe (defined as a single point tool). Quite often, a thread must be cut on a work piece with a single point tool. While thread cutting dies are available, they very often are not profitable for a multitude of reasons. An easy example is where a thread must be cut in ten pieces. It may be a thread where the thread pitch, work piece diameter, thread length and other factors combine to prevent use of certain standard thread cutting dies. Moreover, the number of pieces may be so small as to preclude the cost of programming a numerical control machine. Numerical control machines are intended for high production runs and are otherwise limited in their circumstances. Where they are appropriate, they do a very acceptable job; however, they are limited in application and cannot be used in all circumstances for low volume production runs and the like.
A standard engine lathe is found in practically every machine shop. Numerical control machines and thread cutting dies are not necessarily available in every small machine shop because of cost and, further, in light of their limited application. The present invention is an attachment for a standard engine lathe which converts it into a much more profitable thread cutting machine. The present invention is thus intended for use with an engine lathe of typical construction having ways on which a carriage travels between head stock and tail stock. The carriage normally travels the length of the machine on a lead screw with a traveling nut which engages the lead screw to traverse the machine. Several difficulties in cutting a thread on an engine lathe exist. Some of these will be recounted.
One difficulty in utilizing an engine lathe to cut a thread is that several passes must be made. The first pass is typically fairly deep and removes substantial quantities of metal. However, it normally does not remove enough metal to cut the thread, and, equally important, it may leave a rough surface. Additional passes are required to complete the thread to the required depth and to smooth the faces of the thread to an optimum surface. Two or three problems arise at the termination of the thread. Since the thread is cut with the carriage traversing the ways at the urging of the lead screw, cutting must be ended with rather precisely timed disengagement of the traveling carriage from the lead screw. When this is done, it leaves the tool point at a fixed location with the work piece still rotating and will cut a ring at the end of the thread. In other words, the last few revolutions of the work piece will carve a ring, not a helical thread. Sometimes, it is acceptable to ring a set of threads, but sometimes it is not so desirable. Alternately, thread relief can be cut at the end of the set of threads. This may be limited, however, by abutting shoulders or other factors relating to the geometry of the work piece. Sometimes, the work piece will simply not permit any room because the thread ends immediately adjacent to a protruding shoulder or other portion, thereby limiting maneuverability of the tool point.
Some people attempt to avoid some of these problems by operating the machine at a lower rate of speed. This, however, is not so easily accomplished. If nothing else, it is accomplished at great cost because it extends the machining time for the work piece. In other instances, it is not easily accomplished because the reduction in speed of the work piece and the proportionate reduction of speed in lead screw advance is accomplished at the price of reducing the velocity of the cutting point. Some materials must be cut within specified velocities. While excessive cutting speed may be an obvious drawback, it is not quite so obvious to state that a difficient cutting speed is also undesirable in some instances. If the cutting speed is reduced below the desired range for certain materials, the cutting tool may not cut as smoothly as desired.
Where multiple passes must be made to complete the thread, the several multiple passes are difficult to coordinate so that the operator manually backs off the cross feed mechanism. The present invention overcomes these problems and many others by providing an apparatus which is able to make multiple passes with a single tool point and thereby cut a thread with multiple passes. The cutting point is backed out of the thread at the required location, thereby terminating the thread very nicely, avoiding ringing of the thread, thread runout and otherwise removing it from the work piece to stay clear of the work piece.